The object of the present invention is a method for correcting a counting error in liquid scintillation counting. Liquid scintillation counters are commonly used for counting samples which contain low energy beta or corresponding particles emitting radioactive isotopes such as tritium and carbon-14. The range of the low energy beta particles in the sample is generally at the most a few tens of micrometers. As a consequence, the specimen to be analyzed has to be dissolved into a scintillation liquid, in which the molecules of the isotope to be counted are close enough to the molecules of the scintillation substance so that the beta particles emitted by the isotope to be counted can interact with the molecules of the scintillation substance. In this interaction process a part of the energy of the beta particle is transformed into light, which is converted to an electric pulse generally by means of two photomultiplier tubes which operate in coincidence. The purpose of the coincidence operation is the elimination of the thermal noise of the photomultiplier tubes. The amplitude of said electric pulse is proportional to the energy of the beta particle interacting with the scintillation substance.
Besides said thermal noise there are also other possible errors. The counting error can be caused by both the properties of the apparatus and the quality of the sample. One basic problem which causes counting error is the quenching of the sample, which can be classified into two main categories: i.e. chemical quench and color quench. The chemical quench is a phenomenon, where the solution formed by the specimen and the scintillation substance contains some impurities which reduce the efficiency of the counting system to detect the emitted beta particles by absorbing them. The color quench is a phenomenon, where the solution formed by the specimen and the scintillation substance contains some impurities, which absorb produced scintillation photons. This also leads to the reduction of the counting efficiency.
It is a well-known fact in the field of liquid scintillation counting that the reduction of the counting efficiency due to the chemical and color quenching can be corrected separately by means of quench curves, which describe the relationship between the counting efficiency and the amount of chemical or color quench. The problem arises when samples to be analyzed contain impurities which simultaneously cause chemical and color quench. In the U.S. Pat. No. 4,700,072 is described a method, by means of which the difference of the counting efficiency of the sample to be analyzed compared to the counting efficiency of the sample which contains only chemical quench can be corrected, based on that the colorness of the sample produces a change to the proportion of the pulse amplitudes observed by the photomultiplier tubes. This is a consequence of that the scintillation photons arriving to either photomultiplier tube have to pass a longer path in the coloured solution formed by the specimen and the scintillation substance, whereas photomultiplier tube observes less photons because some of them have been absorbed during the passage.